Various reception problems are encountered in receiving signals from satellites wherein an incoming signal having a frequency of about 2 GHz carries a video signal which is to be recovered. A typical arrangement is shown in FIG. 1. A satellite receiver dish 2 is coupled to a head end processing unit 4 which is mounted close to the dish, out of doors. Unit 2 is coupled via a long coaxial cable 6 to an indoor processing unit 8. Head end processing unit 4 comprises a low noise amplifier 10 and a mixer 12 for mixing the incoming signal with a signal from a local oscillator 14, in order to generate an intermediate frequency (IF) of between 70-612 MHz for transmission along cable 6. An IF of this value permits transmission to the indoor processing unit by cable 6 which is relatively long. The indoor unit 8 comprises an IF amplifier 16, a bandpass filter 18, a further IF amplifier 20, an FM discriminator 22 and a video amplifier 24. Thus the indoor unit filters the incoming IF signal, demodulates the video signal carried thereon in demodulator 22 and provides a video output signal from amplifier 24.
The choice of intermediate frequency is one of the main areas of disagreement between designers, some using a value of 70 MHz since this is an existing standard in microwave link communications and hence the processing techniques are well proven. However, the 612 MHz option has advantages in greater demodulator linearity (due to the lower percentage deviation) and a more simple design (because the inherently higher image rejection makes tracking filters unnecessary). Since it is nowadays possible to make demodulators for frequencies up to 600 MHz at relatively low costs, the 600 MHz option is nowadays preferred.
A further problem arises in fringe reception areas where noise becomes a major problem. In particular, FM demodulators which utilise a frequency discriminator such as a quadrature demodulator exhibit a threshold level below which they are unable to function satisfactorily. The threshold level is the carrier signal to noise signal ratio in the FM signal to be demodulated where the noise begins to introduce characteristic FM noise into the demodulated signal. For FM video signals this characteristic FM noise can produce sparklies into the demodulated video picture and so, degrades the video quality making it unsuitable for most applications.
Hence, threshold extension techniques have been proposed for satellite communcation systems. Threshold extension is a method of extending the carrier signal to noise signal ratio into an increasingly noisy signal whilst maintaining the same quality output signal as obtained from higher carrier to noise signal ratios.
Methods of threshold extension have been proposed employing complex and expensive phase locked loops. Whilst these have operated satisfactorily, they are nevertheless expensive.
My copending application USSN 06/868,908 filed May 29, 1986 entitled Frequency Modulation Receiver Employing Frequency Divider and granted as U.S. Pat. No. 4,806,872 discloses the use of an injection locked oscullator/divider (ILO/D) preceding a quadrature discriminator. An injection locked oscillator/divider is a mixer/oscillator combination whose natural frequency, genrally known as the centre frequency, is equal to or a sub harmonic of the input carrier frequency. The FM signal to be demodulated is injected into the mixer input port of the device and, provided the frequency deviation of the input signal is not excessively high, an oscillator harmonic will tend to phase lock to the FM input signal. The maximum possible deviation in the output signal from the injection locked oscillator/divider centre frequency is proportional to the power of the input signal. Hence, threshold extension is obtained which is proportional to the power of the input signal.